Newly built units are better defenders?

Good daytime!

Playing freeciv i was surprised with following:
I had a city with fortified archer. And another archer was bought
this turn. After ending the turn enemy archer attack my city and
i've got message about my lost archer. I opened city and found out
fortified unit in it.
This was several times, also with warriors.
Is it a feature or i don't understand something main?

Thanks, evyscr.